1. Field
Various examples described herein relate generally to features for limiting axial displacement of relatively rotating portions within Fluid Dynamic Bearing (FDB) motors (e.g., a “limiter”), and in particular relates to FDB motors having lubricated or “wet” limiters.
2. Description of Related Art
Disk drives are capable of storing large amounts of digital data in a relatively small area. Disk drives store information on one or more recording media, which conventionally take the form of circular storage disks (e.g. media) having a plurality of concentric circular recording tracks. A typical disk drive has one or more disks for storing information. This information is written to and read from the disks using read/write heads mounted on actuator arms that are moved from track to track across the surfaces of the disks by an actuator mechanism.
Generally, the disks are mounted on a spindle that is turned by a spindle motor to pass the surfaces of the disks under the read/write heads. The spindle motor generally includes a shaft and a hub, to which one or more discs are attached, and a sleeve defining a bore for the shaft. Permanent magnets attached to the hub interact with a stator winding to rotate the hub and disc. In order to facilitate rotation, one or more bearings are usually disposed between the sleeve and the shaft.
Over the years, storage density has tended to increase, and the size of the storage system has tended to decrease. This trend has lead to greater precision and lower tolerance in the manufacturing and operating of magnetic storage disks. Accordingly, the bearing assembly that supports the hub and storage disk is of increasing importance.
One typical bearing assembly used in such storage systems includes a fluid dynamic bearing. In a fluid dynamic bearing, a lubricating fluid such as air or liquid provides a bearing surface between a fixed member of the housing and a rotating member of the disk hub. In addition to air, typical lubricants include gas, oil, or other fluids. Fluid dynamic bearings spread the bearing surface over a large surface area, as opposed to a ball bearing assembly, which comprises a series of point interfaces. This is desirable because the increased bearing surface reduces wobble or runout between the rotating and fixed members. Further, the use of fluid in the interface area imparts damping effects to the bearing, which helps reduce non-repeatable run-out.
One embodiment of a FDB motor includes a magnetically biased motor. That is, the bearing design cooperates with a magnetically biased circuit or element to establish and maintain fluid pressure in the bearing areas by providing an axial magnetic force, especially in designs where the thrust bearing is defined in the gap at the end of the shaft. The magnetic bias may reduce or eliminate the need to provide hydrodynamic grooves on one or more motor elements in order to accomplish the same, which in turn reduces power consumed by the motor. Typically in such systems, however, the only force or structure holding the rotating portion of the motor in place is the axial magnetic force; therefore, if shock axial forces exceed magnetic forces in the motor, the rotor can shift and the disk drive can become damaged or fail.
Accordingly, FDB spindle motors, and particular, those having electro-magnetic bias and a single thrust bearing, generally include features to limit the axial displacement of the rotating portions relative to the stationary portions during a shock event. Often such features are referred to as a “shock limiter.” A limiter generally limits or reduces the potential for axial displacements of the rotating portions of the motor relative to stationary portions beyond a desired or acceptable range of axial motion. A limiter may be fixed to or form part of a rotating member and separated by a small gap from a stationary portion of the motor. During a sufficiently large shock event the limiter will move axially and contact the stationary portion of the motor, thereby limiting the axial motion of the rotational portion of the motor.
If a shock event occurs during operation, however, the limiter feature will place rotating portions in direct contact with stationary portions. The contacting surfaces may lead to particle generation or gall and lock-up of the motor during contact, in part, because the contacting surfaces are not lubricated, e.g., a dry surface-to-surface contact. Particle generation may result in reduced performance or failure of the spindle motor or disc drive, e.g., contaminating bearing fluid or the like.